1. Field of the Invention
The present invention relates to a temperature control method and apparatus therefor, suitable for controlling the heating of an object such as a semiconductor substrate (hereinafter referred to as "wafer") in a heating furnace provided with radiant heat sources.
2. Description of the Prior Art
In the process of manufacturing a semiconductor device, heat treatment is applied to a wafer to achieve several objects, such as uniformity of composition in ion implanation layers, stabilization of silicon films, and formation of ohmic contacts. In such heat treatment, the wafer must be rapidly and uniformly heated according to a predetermined target temperature variation over time. After the temperature of the wafer reaches a target heat treatment level, the wafer must be maintained at that temperature.
Typically, a heating furnace provided with radiant heat sources such as strong halogen lamps is employed for such heat treatment of a wafer or the like. Feedback control is provided, in which the temperature in the heating furnace is detected and any deviation between the detected temperature and a target temperature value is calculated, whereby the amount of electric power supplied to the radiant heat sources is controlled so as to decrease the deviation.
FIG. 1 is a block diagram illustrating a conventional prior art heat treatment apparatus employing such a temperature control method. Referring to FIG. 1, a wafer 1 to be heated is introduced into a heating furnace 3 provided on both sides of inner surfaces thereof with a plurality of radiant heat sources 2, and the wafer 1 is supported by a wafer supporting mechanism 4.
On the other hand, at a target temperature setter 10, a trapezoidal target temperature change curve F.sub.1 (T.sub.0), as illustrated in FIG. 2, is previously set as a function of time t. Referring to FIG. 2, the temperature is expressed in a scale of absolute temperature [K], and symbol T.sub.p indicates the stationary treating temperature. A target temperature signal T.sub.0 outputted from the target temperature setter 10 in FIG. 1 in response to the target temperature change curve F.sub.1 (T.sub.0) is supplied to a subtracter 11 with an actual temperature signal T.sub.r detected by a temperature detector 5 such as a thermocouple provided in the heating furnace 3.
The subtracter 11 obtains deviation .DELTA.T between T.sub.0 and T.sub.r by the following expression (1), to output a deviation signal .DELTA.T to multipliers 13 to 15 in a PID controller 12: EQU .DELTA.T=T.sub.0 -T.sub.r ( 1)
The multiplier 13 multiplies the deviation .DELTA.T by a constant K.sub.1 to provide a deviation proportional signal S.sub.p. The multiplier 14 multiplies the deviation .DELTA.T by a constant K.sub.2 and the product thus obtained is integrated with time in an integrator 16 of a subsequent stage, thereby to provide a deviation integrated signal S.sub.I. The multiplier 15 multiplies the deviation .DELTA.T by a constant K.sub.3 to differentiate the product by time in a differentiator 17 of a subsequent stage, thereby to provide a deviation differentiated signal S.sub.D. These signals S.sub.p, S.sub.I and S.sub.D are added up by an adder 18, to provide a temperature control signal S.sub.T.
The temperature control signal S.sub.T is supplied to a power supply unit 19 for supplying electric power P to the radiant heat sources 2. The power supply unit 19 is provided with a power controller 21 and a power source 20. The temperature control signal S.sub.T is inputted in the power controller 21, which controls the electric power P supplied from the power source 20 to the radiant heat sources 2 to be proportional to the temperature control signal S.sub.T. Thus, the temperature control signal S.sub.T itself functions as a power control signal in such a conventional apparatus.
Thus, radiant energy from the radiant heat sources 2 is increased or decreased in response to the deviation .DELTA.T, to control the temperature of the wafer 1 along the target temperature change curve F.sub.1 (T.sub.0).
In such a conventional temperature control method, however, the temperature in the heating furnace 3 cannot effectively respond to a drastic rise in the target temperature. This problem is now described in further detail with reference to FIG. 3.
In FIG. 3, the left half of FIG. 2 is shown enlarged, to illustrate respective time change curves F.sub.2 (T.sub.r) and F.sub.3 (P) of the actual temperature T.sub.r in the heating furnace 3 and the supplied electric power P. As is obvious from FIG. 3, when the target temperature T.sub.0 drastically rises, and the temperature in the heating furnace 3 must be rapidly raised, for example, from a range of 200.degree. C. to 300.degree. C. to more than 1000.degree. C. in several seconds, a considerable time is required until the actual temperature T.sub.r in the heating furnace 3 reaches the stationary treating temperature T.sub.p and is stabilized at that level. Thus, it is difficult to accurately perform a desired heat treatment.
In order to solve the problem, the constants K.sub.1, K.sub.2 and K.sub.3 corresponding to feedback gains may be increased. However, in this method, the actual temperature T.sub.r may overshoot or hunt around at time t.sub.1 (FIG. 3) at which the temperature control process advances from a temperature increasing step to a temperature maintaining step. Thus, the aforementioned problems cannot be adequately solved by such a method.
Futhermore, in the conventional method, temperature control is performed on the premise that substantially all of the electric power P supplied to the radiant heat sources 2 is converted into radiant heat. Such a premise is not true as hereinafter described, and leads to errors in exercising temperature control of the wafer in the temperature increasing step.
The aforementioned problems are not restricted only to heat treatment of wafers, but are often encountered in controllably heating arbitrary objects by radiant heat sources.